A Commentary on Clearance-to-Combustibles and Other Issues Relating to Masonry Fireplaces and Chimneys

by Jim Buckley, 10/8/00

Within a year or two all 50 states and most local city and county building authorities will adopt the new International Building Codes. The BOCA code, UBC, Southern Building Code and the CABO One and Two Family Dwelling Code are all dead. The committees that sponsored these codes have merged into the International Code Council (ICC) and the old codes will not be revised.

It's good news for masonry fireplaces and chimneys in several ways:

Family of 2000 International Codes Available

The family of 2000 International Codes are available by calling any of the code organizations or through the ICC on line at "". The 2001 code is not yeat available and the 2002 proposed changes are due November 1, 2000 but these are mainly clean-up, editorial and minor changes.

Masonry fireplace and chimney issues are in pretty good shape in both the new International Building Code (IBC) and the Internatioanl Residential Code (IRC) as a result of the code change committee meetings held in Costa Mesa, CA in the spring of 1999 and consolodated in Birmingham last spring.

Code officials and industry associations had been working on the new codes for several years. Sweeping changes were made to the fireplace and chimney sections of the residential code in 1995 when the CABO One and Two Family Dwelling Code was reorganized along the lines of the National Fire Protection Association 211 Code. To that mix the seismic provisions of the UBC, the wind load provisions of the SBCCI and the freeze/thaw provisions of the BOCA codes were added. These sweeping changes were then in 1998 incorporated into the fireplace and chimney sections of the new International Residential Code (IRC).

IBC Chapter 21 Rewritten

The International Building Code (IBC), which is used mainly for non residential buildings but does have a section on fireplaces and chimneys needed to be coordinated with the IRC. At the Costa Mesa meeting in 1999 the IBC Committee approved a complete re-organization of the sections of Chapter 21 dealing with masonry fireplaces and chimneys to reflect the IRC in structure. The Committee also approved adding a section on masonry heaters to the IBC code for the first time, as well as all the ASTM references, seismic and flue sizing language we've been working on for the last few years in the IRC arena. It was an important victory crafted within the Masonry Alliance for Codes and Standards (MACS) with support from the Brick Industry Association (BIA), the National Concrete Masonry Association (NCMA), the Portland Cement Association (PCA), the Clay Flue Lining Institute (CFLI), the Masonry Heater Association (MHA), the National Fire Protection Association (NFPA), the Hearth Products Association (HPA) and the National Chimney Sweep Guild (NCSG) among others.

Clearance to Combustibles

The clearance to combustibles issue, however, had still to be worked out. Later in the week, while other code changes were being addressed, we hammered out an agreement on the clearance to combustibles issues. All the codes - the BOCA, UBC, One and Two Family, even the NFPA 211 - have long been ambiguous and even self contradictory on the distance to combustibles issue (see background below).

Five years ago in Nashville, in the effort to combine all these codes into one, some of this ambiguity was resolved but in such a way that the home builders felt they couldn't continue to abut drywall, exterior sheathing or wood trim to masonry chimneys ("combustible framing" was changed to "combustible materials"). The National Home Builders Association (NHBA) said the code was "un-buildable" if the only way to close off the air space around masonry fireplaces and chimneys is with sheet metal. The previous year the NHBA changed the wording in the second draft of the IRC to:

"Sheathing materials shall be permitted to abut the masonry chimney walls. The juncture of the sheathing materials and the chimney walls shall be sealed by methods approved by the code official."

While this language essentially re-stated the way builders have been closing off masonry chimneys all along, the vague and permissive language elicited objections in the form of eight code change proposals from a wide range of safety minded organizations - all of which (except the MACS proposal) would have eliminated all combustible material in contact with masonry fireplace and chimneys. As the NHBA spokesman pointed out, these proposals left no acceptable way to trim around the masonry. "How are we going to keep out the bats and the rats?" he complained.

For three days and nights Jim Brewer (NCSG), Chip Clark (BIA), Jerry Frisch (MHA and myself, representing MACS, worked on language. We might not have come to an agreement except that, with three homebuilders on the IRC Committee, not agreeing would likely mean the permissive NAHB language would stay in the code. In the end we did all agree and we secured the support of all the other code change proponents on this issue and worked with the NHBA so that their opposition was only half hearted.

Here then is what the new International Residential Code (IRC) says about clearance to combustibles around masonry fireplaces and chimneys:

R1001.15 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1001.16.

And in the Fireplace Section:

R1003.12 Fireplace clearance. All wood beams, joists, studs, and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The air space shall not be filled, except to provide fire blocking in accordance with Section R1003.14.

Historic Implications

In order to develop support for allowing combustible material to be in contact with masonry fireplaces and chimneys with thickened walls we built a masonry fireplace and tested it to the clearance to combustibles portion of UL 127 - the fireplace standard. The results summarized at showed that increasing the wall thickness to 12" of masonry at the sides of a fireplace was at least as safe as a "known to be safe" condition. We would like to do more testing and to review other tests, many of which are proprietary, and study fire statistics, but this could lead to code changes important for historic properties. A hundred years ago builders knew that chimneys that were part of thick masonry walls were safe. They also knew that chimneys that were increased in thickness as they passed through combustible floors and roofs were safe (See Victorian Western Barrel Chimneys). Yet modern codes don't recognize these time honored practices. Our current efforts to change the modern code to allow an increased thickness of masonry in place of an air space clearance to combustible material will make many historic chimneys legal and open up new more appropriate ways to bring historic chimneys up to modern code standards.

ASTM Standards Referenced

Of particular interest to flue liner manufactures the new codes will reference ASTM C315, the flue lining standard and ASTM C 1283, the clay flue installation standard and chimney construction in seismic areas was also rationalized.

The Uniform Building code (UBC) adopted by most western states has not referenced ASTM C315 and has allowed reinforcing bars to be grouted in solid between the flues and the masonry chimney. "Seismic zones" have been replaced by "seismic design categories" that depend on new detailed maps and soil conditions. Seismic requirements are definitely moving east. The new language in both Chapter Ten of the IRC and Chapter 21 of the IBC will require an air space around the flues in accordance with ASTM C315 even when seismic reinforcing is required. (See seismic requirements below.)

The code also references ASTM C 27 or C 1261 for firebrick and requires that firebrick be laid with medium-duty refractory mortar conforming to ASTM C 199.

Masonry heaters were included in the code for the first time with a reference to ASTM E 1602. For the actual code language check below.

Exterior Air Supply

We have so far failed to come to an agreement on the exterior air supply issue. The NFPA 211 representative as well as those representing the HPA and the NCSG wanted to eliminate or make the exterior air supply optional while the BIA and the NAHB representatives wanted more research. So the IRC Committee suggested we get it together for next year and disapproved all four proposals dealing with the exterior air supply for fireplaces issue.

The whole concept of requiring an exterior source of combustion air for fireplaces is flawed and has led to some unsafe practices. Three of the four proposed changes to the IRC would have made the provision of exterior combustion air optional and only permitted if certain safety rules to prevent back drafting were followed.

The value of exterior air supplied directly to the firebox of a fireplace has never been tested. Many say it simply doesn't work. At best the six square inches required is insufficient for an open fireplace. And it can lead to bad smells, blowing ashes, enough turbulence to make the fireplace smoke and, if it can back draft as in a positively pressured space such as an upstairs bedroom, it can be a fire hazard.

There are those who feel that the exterior air supply is needed to keep the fireplace cool in the event glass doors are added to the fireplace. In any event, since we couldn't agree among ourselves, the IRC Committee left the code language as it was, requiring exterior air supply.

2000 International Code Seminal

The International Building Code (IBC) and the International Residential Code (IRC) published last spring as the 2000 International Code are now available and will soon be adopted nationally. The code change process goes on, of course, and there will be opportunities to propose changes in future years, but the 2000 International Code, and all the work it represents, is seminal.

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Possible Side Bars

Clearance To Combustibles Background

In the most comprehensive re-writing of the code since the 1940s, Chapter 10 of the IRC and Chapter 21 of the IBC, dealing with fireplaces and chimneys, takes an encompassing view of building styles and construction methods practiced all over the country, and also addresses some of the ways fireplaces and chimneys used to be built before the post World War II building boom and continue to be built in some custom homes and restorations.

The old (current) codes were promulgated during the unprecedented building boom after WWII and focused narrowly on the building methods commonly used then - mostly wood-frame construction. It was assumed that masonry chimneys would be built in houses built of wood, and the codes generally required a 2-in. clearance, or air space, to combustibles.

Tests showed that, while masonry takes a long time to heat up, it isn't a very good insulator and with steady operating temperatures of 1,000 degrees F or more the temperature on the outside surface of the chimney would never reach an acceptable equilibrium and would keep on getting hotter. The idea of the air space was to have some air circulating around the masonry to cool it. You wouldn't want to enclose the chimney within combustible materials or have combustible materials in contact with large areas of the surface. And you wouldn't want to seal up and insulate the air space, which would interfere with the cooling air circulation, but the edges (not the flat sides) of "sheet materials", like siding, flooring and drywall, could safely touch the masonry, some said.

None of this was very clearly spelled out in any of the model codes but in practice code officials allowed builders to close the gap between a frame wall and the masonry chimney with sheet materials such as drywall and siding. Most allowed the "air space" within the wall to be insulated but this practice was deplored by other officials.

In recent years these old arguments and the "intent of the code" have all but been forgotten. In some areas of the country building officials who have worked their profession for ten years may never have inspected a masonry chimney and when asked to do so (guess what?) they read the code.

Unfortunately the codes are poorly written, self contradictory and unclear. No combustible materials within 2" of the chimney is what the codes say. So how do you close the gap between the wall and the chimney or around the hearth? With sheet metal? How many living rooms have you seen with sheet metal trim around the mantel? And in the next paragraph the mantel, it says, must be 6" away from the fireplace opening - but presumably 6" away it is in contact with the masonry. What about the clearance to combustibles or are wooden mantels not "combustible"?

And how does that distance-to-combustibles rule apply to a chimney in a masonry wall? A few companies are promoting brick or block residential wall systems. We've heard of more than one official who wouldn't allow a combustible window or door frame several feet away from the chimney unless there was a 2" air space somewhere between the chimney and the combustible framing. The rule may be silly but usually the builder just puts a metal fireplace and metal chimney within a wooden chase on his masonry house rather than deal with the patched and ambiguous language governing masonry chimneys and fireplaces. (Wooden chimneys on masonry houses! It should be illegal.)

The new International Residential Code (IRC) clearly restates the basic clearance to combustibles rules but would permit masonry fireplaces and chimneys to be in contact with combustibles in specific situations expressed as exceptions. See the new code language above.

While the new clearance to combustible language is much better than the current linguistic ambiguity and the resultant policy of letting builders and inspectors figure it out in the field, most of us would like to see some testing or at least heat transfer calculations. Many tests have been performed on code minimum masonry chimneys (a clay flue lining surrounded by 4" on masonry) completely wrapped in a plywood enclosure as specified by UL 1777. The masonry chimneys don't pass, meaning the combustible plywood gets hotter than the allowable 90 degrees F above ambient temperature, unless the flue lining is insulated. But what if only the edge of the plywood sheeting touches the chimney? Or maybe 6" of combustible trim only touching on the sides of the chimney 8" or 12" away from the flue? It would stand to reason that a masonry chimney with its surface mostly exposed would dissipate more heat and not get as hot as a totally enclosed chimney. But would it be too hot? How about a bigger chimney with more mass than the minimal chimney - one that is five feet wide with three flues or one that is part of a masonry wall?

These are not new issues. In a larger historical context, modern codes have narrowly focused on the kind of production frame houses built after WWII. Older and historic masonry chimneys, especially the ones in better houses, often contained three or four flues or were built into thick solid masonry walls. For safety many old chimneys were 8" thick or corbeled out so the chimney walls were increased in thickness to 8" where they passed through combustible floors and roofs. These were long standing practices based on experience. It would be nice to re-examine some of these methods of building safe masonry chimneys, not only to help us maintain and restore these older chimneys but also because some new houses are being well built using masonry and traditional building techniques.

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Rumford Fireplaces and Masonry Heaters

The IBC and the IRC now both permit Rumford fireplaces and masonry heaters and provide guidelines about their construction and safety.

Rumford fireplaces were so common in the 19th and early 20th century that "Rumford" was synonymous with "fireplace" and Thoreau listed Rumfords along with plaster walls and venetian blinds as luxuries a civilized man could take for granted. Masonry heaters, a tradition even older than Rumford fireplaces, are culturally important features in the homes built by Scandinavians, Germans, and Russians. Both Rumfords and masonry heaters were not clearly permitted by the codes developed with the decorative but ineffective fireplace of the 1950s in mind. The IBC and the IRC now specifically permit the construction and restoration of these efficient and clean-burning Rumford fireplaces and masonry heaters. Rumford fireplaces have been specifically permitted in the BOCA and UBC codes for several years but the code issues involved in building masonry heaters have never before been addressed in any of the model codes. Here is the language:


R1001.11 Firebox dimensions. The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 inches (508 mm). The throat shall not be less than 8 inches (203 mm) above the fireplace opening. The throat opening shall not be less than 4 inches (102 mm) in depth. The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall not be less than the cross-sectional area of the flue.


R1005.1 Definition. A masonry heater is a heating appliance constructed of concrete or solid masonry, hereinafter referred to as masonry, having a mass of at least 800 kg (1760 lbs.), excluding the chimney and foundation, which is designed to absorb and store heat from a solid fuel fire built in the firebox by routing the exhaust gases through internal heat exchange channels in which the flow path downstream of the firebox includes at least one 180 degree change in flow direction before entering the chimney and which delivers heat by radiation from the masonry surface of the heater which shall not exceed 230 degrees F (110 degrees C) except within 8 inches (203 mm) surrounding the fuel loading door(s).

R1005.2 Installation. Masonry Heaters shall be listed or installed in accordance to ASTM E-1602

R1005.3 Seismic reinforcing. Seismic reinforcing shall not be required within the body of a masonry heater whose height is equal to or less than 2.5 times it's body width and where the masonry chimney serving the heater is not supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section R1006.

R1005.4 Masonry heater clearance. Wood or other combustible framing shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater, provided the wall thickness of the firebox is not less than 8 inches (203 mm) and the wall thickness of the heat exchange channels is not less than 5 inches (127 mm). A clearance of at least 8 inches (203 mm) shall be provided between the gas tight capping slab of the heater and a combustible ceiling. The required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.

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Fire Safety Lessons

The late 1970s and early 1980s saw many chimney fires caused by air-tight wood stoves and inserts inappropriately installed or improperly used by many of us following the energy crisis a few years earlier. As one might expect, regulations followed the crisis and in the next few years various standards were developed to specify how these appliances should be installed and used. Now, after the dust has settled, the best of these standards - a positive connection to the flue and UL listed relining systems - have been incorporated, either directly in the IBC and IRC or by reference. We have learned from the rash of fires in the '70s and '80's and the new code will result in safer building practices.

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Seismic and Wind Load Design Issues

The new model codes also make more sense out of seismic and wind load design issues that were often unclear, self-contradictory or at least varied widely from one model code to another.

In most of the country, for example, clay flues were installed with an air space around them for expansion, but in the earthquake-prone areas of the west that space was often filled with reinforcing bars and grouted solid. Then again, maybe not. In California, builders and inspectors worried about thermal expansion and the code didn't specifically say the steel had to be grouted. Lots of the chimneys that fell over in the recent California quakes were not grouted nor attached to the house as required by code - sort of.

The new IRC will fix all that. The new code preserves the air space for expansion and, at the same time, is clear about proper reinforcing and grout placement where required.

R1006.3 Seismic reinforcing. Masonry or concrete chimneys shall be constructed, anchored, supported and reinforced as required in this chapter. In Seismic Design Category D-1 and D-2, masonry and concrete chimneys shall be reinforced and anchored as detailed in Section R1006.3.1 and R1006.3.2. In Seismic Design Categories A, B or C, reinforcement and seismic anchorage is not required. In Seismic Design Categories E and F, masonry and concrete chimneys shall be reinforced in accordance with the requirements of Section 2101 through 2109.

R1006.3.1 Vertical reinforcing. For chimneys up to 40 inches (1016 mm) wide, four No. 4 continuous vertical bars, anchored in the foundation, shall be placed in the concrete, or between wythes of solid masonry, or within the cells of hollow unit masonry, and grouted in accordance with Section R609. Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion. For chimneys greater than 40 inches (1016 mm) wide, two additional No. 4 vertical bars shall be provided for each additional 40 inches (1016 mm) in width or fraction thereof.

R1006.3.2 Horizontal reinforcing. Vertical reinforcement shall be placed enclosed within 1/4-inch ties, or other reinforcing of equivalent net cross-sectional area, spaced not to exceed 18-inches on center in concrete, or placed in the bed joints of unit masonry, at a minimum of every 18 inches (457 mm) of vertical height. Two such ties shall be provided at each bend in the vertical bars.

R1006.4 Seismic anchorage. Masonry and concrete chimneys and foundations in Seismic Design Category D shall be anchored at each floor, ceiling or roof line more than 6 feet (1829 mm) above grade, except where constructed completely within the exterior walls. Anchorage shall conform to the following requirements:

R1006.4.1 Anchorage. Two 3/16-inch by 1-inch (4.8 mm by 25 mm) straps shall be embedded a minimum of 12 inches (305 mm) into the chimney. Straps shall be hooked around the outer bars and extend 6 inches (153 mm) beyond the bend. Each strap shall be fastened to a minimum of four floor joists with two 1/2-inch (12.7 mm) bolts.

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